how long to spend in Nice?

Honestly - I wouldn't spend any time there at all. While I'm sure you could find something to do and see there (like in any big city), I don't know of anything so special about Nice that makes it worth trading a day somewhere else. I'd spend the day elsewhere, even if it's just "downtime". If you have three weeks, and your plan is to do Paris (and other bits of France?) and a good chunk of Italy, you probably don't have much extra time to waste. Why does your friend seem so intent on going there?

Nice was a must for me. I'd say 2 nights as I hate one-night stays. The Chagall and Matisse museums are both wonderful if you're interested in their art. The promenade is beautiful and there are many terrific seafood restaurants. I also enjoyed visiting the market. Of course, you can also relax and hang out, too.

Bethanie, Nice is a great "home base" for touring in that area, and I'd suggest at least three nights there (depending on your interests of course). In addition to the Museums that Andrea mentioned, the "old town" (very close to the Promenade des Anglais has great street markets, street entertainers and good restaurants. You could also take day trips to Villefranche, Eze and of course Monaco and Monte Carlo. The Changing of the Guard takes place at the Royal Palace at 11:55 every day (as I recall) and the Oceanographic Museum is fantastic. If you're planning to visit the famous Casino, there IS a dress code, so keep that in mind. The beach in Nice is a bit "rocky" so not really well suited for sunning. However, I believe it's possible to rent lounge chairs? I've found that Nice is a great place to "unwind" from strenuous travels in other areas. There's lots to do, but it's also a good place to just relax if you're not in the mood to do a lot of touring. Happy travels!

Bethanie Have a look at the RS books. They will show lots and lots to do there and homebasing. Walk up to the jardin exotique above Eze Village (good hiking shoes please). A photo from there is the background of one of my laptops. Walk the coast walk around Cap Ferrat. Oh my! A photo from there is the background of another laptop. Enjoy walking the many levels of Monaco. A photo of the fountain at dusk outside the Casino is the background of my desktop. Take plenty of photo cards with you. And make sure your camera battery is charged before you take your walk (voice of experience). When all else fails a camera phone can help (poorly).

What Ken said. 1 Euro each way buys a bus ticket to any town close. We liked Grasse for the perfumes and scenery. Eze was beautiful and Monte Carlo was ok. Not really impressed with it and really expensive. While in Nice, be really careful of cash, passport, etc. Keep in under clothing pouch!! Pickpockets abound in the train station and on the tram and at tram stops.

Thanks so much, everyone! I love Matisse and I completely forgot about the museum in Nice! Lovely. It sounds like there are a lot of nice things there, for a variety of interests. Sounds good since I will be dragging her (my friend) through big museums and churches in all the other places we are visiting :) (she does like art, too, though...though I suspect not as much as me).

Bethanie - If you like Renoir, his home and studio is in Cagnes sur Mer, a short bus ride from Nice.

there are a few nice old villages on the hill you can day trip to. The long distance bus leaves in the morning take you up the hill you can do 1 or 2 villages in a day. it's a nice change from beaches and museums.

When we took RS tour of "Paris and the south of France" in 2009, our tour ended up in Nice. We were there 3 nights and did not get to see everything we wanted to see, especially day trips. Your should really prioritize the cities you want to see then assign the number of days. You could easily spend 3 weeks in Paris, Venice, Florence, and Rome and leave Nice for another trip especially considering you will spend 3 days traveling between those four places. Happy travels